1. Field of the Invention
This invention generally relates to a surface acoustic wave device that employs a piezoelectric material, and more particularly, to a surface acoustic wave device having multiple interdigital transducers (hereinafter simply referred to as IDT) on a piezoelectric material substrate (hereinafter simply referred to as piezoelectric substrate).
2. Description of the Related Art
In these years, the above-mentioned type of filter, which is composed of surface acoustic wave (hereinafter referred to as SAW) device having multiple IDTs on the piezoelectric substrate, has been employed for a bandpass filter in a television set having a frequency range of 30 MHz to 400 MHz and an RF filter in a mobile telephone having a frequency range of 800 MHz to several GHz. An IDT includes a pair of comb-like electrodes. Each comb-like electrode is composed of a bus bar and electrode fingers having first edges connected to the bus bar and second edges that are open. A pair of comb-like electrodes is arranged so that the electrode fingers of the comb-like electrodes are alternately crossed or interleaved at regular intervals. In other words, the interleaved electrode fingers are alternately connected to two bus bars. A SAW is generated by applying an alternating voltage across the pair of comb-like electrodes. The SAW has a frequency response by which a filter having a desired frequency characteristic is obtainable.
FIG. 1 shows a filter with the SAW. Japanese Patent Application Publication No. 10-41778 (hereinafter referred to as Document 1) discloses this type of filter. Referring to FIG. 1, there are arranged a first IDT 10, a ground electrode 20, and a second IDT 30 on a piezoelectric substrate 1. The first IDT 10, the ground electrode 20, and the second IDT 30 are adjacently arranged in a direction of the SAW propagation. The ground electrode 20 is arranged between the first IDT 10 and the second IDT 30, serving as a shield electrode. The first IDT 10 serves as an input electrode (or output electrode) and the second IDT 30 serves as an output electrode (or input electrode). The ground electrode 20 prevents electromagnetic coupling of the IDT 10 and the IDT 30. Also, the ground electrode 20 is arranged on a tilt in order to prevent the SAW that travels from the IDT 10 (or the IDT 30) from being reflected by the ground electrode 20 and returning to the IDT 10 (or the IDT 30).
The IDT 10 includes a pair of comb-like electrode 10a and 10b. The comb-like electrode 10a includes a bus bar 12a and multiple electrode fingers 14a. The comb-like electrode 10b also includes a bus bar 12b and multiple electrode fingers 14b. The open edges of the electrode fingers 14a face those of the electrode fingers 14b, which are referred to as crossing portions or overlapping parts. The crossing portions of the interleaved electrode fingers that face each other are involved in excitation of SAW. As shown in FIG. 1, an electrode finger pattern is weighted. The electrode finger pattern is defined as a pattern formed by the electrode fingers. The electrode finger pattern may be weighted by, for example, apodization. By this apodization, lengths of the electrode fingers in the overlapping parts (hereinafter referred to as aperture length) vary in the propagation direction. The aperture lengths are relatively small in the vicinity of both sides of the IDT 10, which is defined as small overlapping parts. On the other hand, the aperture lengths are relatively large around the center of the IDT 10. The aperture length is proportional to excitation intensity. Therefore, the strong SAWs are generated around the center of the IDT 10, and weak SAWs are generated in the vicinity of both ends of the IDT 10. The frequency characteristic may be altered by changing the weight by apodization.
The IDT 30 also includes a pair of comb-like electrodes. However, the IDT 30 is not weighted, which is different from the IDT 10. In other words, the electrode fingers of the IDT 30 have an identical overlapping length. The above-mentioned IDT is defined as a normal IDT.
The bus bar 12a is connected to an electrode pad 15, and the bus bar 12b is connected to an electrode pad 16. The bus bars of the IDT 30 are respectively connected to electrode pads 17 and 18. Thus, the filter with the above-mentioned configuration serves as a bandpass filter.
With the above-mentioned SAW device, it is necessary to consider a power-flow angle of the piezoelectric substrate 1. The power-flow angle defines the propagation direction of the SAW. As shown in FIG. 1, the power-flow angle is created by an X-axis and the propagation direction of the SAW, where the X-axis is defined as the direction parallel to the central axes of the longer sides of the IDTs 10 and 30, and a Y-axis is defined as the direction perpendicular to the X-axis. The power-flow angle is specific to the piezoelectric materials, and generally ranges from zero to a few degrees. For example, 112° LiTaO3 has the power-flow angle of a few degrees. FIG. 1 shows a case where the power-flow angle of the piezoelectric substrate 1 is not zero. The SAW travels from the IDT 10 at the power-flow angle. Therefore, the IDT 30 is unable to receive the entire SAW. The SAW that is not received by the IDT 30 is defined as leaked wave, which degrades the stopband characteristic.
The above-mentioned drawback has been well known, and some proposals have been made. International Publication Number WO 96/10293 (hereinafter referred to as Document 2), Japanese Patent Application Publication No. 10-209802 (hereinafter referred to as Document 3), and Japanese Patent Application Publication No. 11-205079 (hereinafter referred to as Document 4) have proposed that, in the case where the piezoelectric substrate having a non-zero power-flow angle, the electrodes are arranged so that the propagation direction of the SAW may be parallel to the power-flow angle. This is shown in FIG. 2. In addition, Japanese Patent Application Publication No. 53-114644 (hereinafter referred to as Document 5) has proposed that the aperture length of the output electrode is designed to be greater than that of the input electrode so that the leaked wave caused by the power-flow angle may be adjusted. This is shown in FIG. 3. Referring to FIG. 3, the IDT 30 extends from both sides of the IDT 10, by a width A, in the direction perpendicular to the central axes. Similarly, the ground electrode 20 also extends from both sides of the IDT 10, by the width A, in the direction perpendicular to the central axes. In Document 5, the aperture lengths of the output electrode in the Y direction are 1.05 to 1.50 times those of the input electrode.
However, with the above-mentioned techniques, it is to be noted that a larger piezoelectric substrate is required to arrange the IDT 10 on a tilt as disclosed in Documents 2, 3, and 4, or to arrange the IDT 30 and the ground electrode 20 having larger aperture lengths as disclosed in Document 5. The above-mentioned techniques cause a problem that the SAW device cannot be downsized. In particular, if the aperture lengths are made relatively large as disclosed in Document 5, the aperture lengths become larger than necessary, and the electrode finger resistance is increased. This may increase losses.